An accurate knowledge of the hosts of hematophagous arthropods is essential in studies of diseases transmitted by arthropods. Identification of blood meals of these arthropods requires a test which is sensitive enough to detect partially digested blood and specific enough to identify the various hosts. Our study is concerned with a method which involves crystallization of certain blood components in the blood samples from the arthropod midgut and comparing the crystal structure with that of known material. One of the original objectives was to develop the crystallization technique as a supplementary method for separating blood samples of closely related forms which can be separated only with difficulty utilizing serological techniques. With NIH support (AI 10529-02) during the past year and a half, this objective has been attained. In almost all instances where hemoglobin crystals were prepared, morphologic differences were sufficient to permit ready separation of one animal from another. Furthermore, our ability to consistently produce distinct crystals from non-primate mammals has enabled us to construct a dichotomous key to identify almost all non-primata mammalian blood samples. The only obstacle to developing an identification system applicable to all animals has been the inconsistency in crystallizing primate and bird blood samples. The objective of this study is to (1) utilize information on non- primate mammals and test the field applicability of the technique in an area where a variety of biting flies (mosquitoes, Phlebotomine sandflies and Culicoides midges) are known to feed on non-primate mammals (i.e., Panama); (2) develop a more suitable technique for obtaining hemoglobin crystals from primates, birds and reptiles-amphibians so that the technique can be used as an independent method for host blood meal identification among hematophagous arthropods which feed on a wide variety of hosts.